Cathode for fuel cell having two kinds of water-repellency and method of preparing the same and membrane electrode assembly and fuel cell comprising the same

ABSTRACT

A cathode for a fuel cell includes a gas diffusion layer contacting with a separator having a channel and a catalyst layer interposed between the gas diffusion layer and an electrolyte membrane. The catalyst layer of the cathode has two portions with different water-repelling properties, and a portion of the catalyst layer that does not face a channel has a higher water-repelling property than a portion that faces a channel. This cathode controls water-repelling property of the catalyst layer differently according to locations, so it is possible to keep an amount of moisture in an electrode in a suitable way and to restrain generation of flooding, thereby improving the performance of the cell.

TECHNICAL FIELD

The present invention relates to a cathode for a fuel cell having twokinds of water-repellencies, a method for preparing the same, and amembrane electrode assembly and a fuel cell having the same, and moreparticularly to a cathode for a fuel cell capable of controllingwater-repellency of a catalyst layer, a method for preparing the same,and a membrane electrode assembly and a fuel cell having the same.

BACKGROUND ART

Recently, as depletion of conventional energy resources such as oil orcoal is foreseen, interest in an alternative energy is increasing. Afuel cell is one of the alternative energy, and advantageously has ahigh efficiency, does not emit pollutants of NO_(X) and SO_(X) and usesa fuel that is abundant in quantity, and therefore, the fuel cellattracts public attention.

The fuel cell is a power generation system that converts chemical energyof a fuel and an oxidant to electrical energy, and typically hydrogenand hydrocarbon, for example methanol or butane is used as a fuel, andoxygen is used as an oxidant.

In the fuel cell, a membrane electrode assembly (MEA) is the basic unitfor generating electricity, and includes an electrolyte membrane andanode and cathode electrodes formed at opposite sides of the electrolytemembrane. FIG. 1 illustrates the principle for generating electricity ofa fuel cell, and Chemical FIG. 1 represents a reaction formula of a fuelcell in the case that hydrogen is used as a fuel. Referring to FIG. 1and Chemical FIG. 1, an oxidation reaction of a fuel occurs at an anodeelectrode to generate hydrogen ions and electrons, and the hydrogen ionsmove to a cathode electrode through an electrolyte membrane. Thehydrogen ions transmitted through the electrolyte membrane and theelectrons react with oxygen (oxidant) at the cathode electrode togenerate water. This reaction causes the electrons to move to anexternal circuit.

Anode electrode: H₂→2H⁺+2e ⁻

cathode electrode: ½O₂+2H⁺+2e ⁻→H₂O

Reaction formula: H₂+½O₂→H₂O  [Chemistry Figure 1]

FIG. 2 illustrates a general configuration of a membrane electrodeassembly for a fuel cell. Referring to FIG. 2, a membrane electrodeassembly for a fuel cell includes an electrolyte membrane, and an anodeelectrode and a cathode electrode located at the opposite sides of theelectrolyte membrane. The anode and cathode electrodes respectivelyinclude a catalyst layer and a gas diffusion layer. The gas diffusionlayer includes an electrode substrate and a microporous layer formed onthe electrode substrate.

In the fuel cell, the moisture that is also a resultant material ofreaction in an electrode assists ion transfer, but an excessive amountof moisture may block the micro pores in the catalyst layer or the gasdiffusion layer. Namely, the moisture discharging ability in theelectrode surface of a fuel cell is a factor determining performance ofthe cell. Thus, the moisture introduced into the electrode or themoisture generated in the electrode should be controlled suitably. Ifmoisture is not discharged suitably, flooding occurs, which decreasesthe three-phase reaction sites and reduces an activation area ofcatalyst, thereby deteriorating the efficiency of the fuel cell.

However, a catalyst layer of a conventional fuel cell membrane electrodeassembly is made by coating with one kind of ink including a catalystand an ionomer, so the same catalyst layer is formed entirely. Thus, itwas difficult to control water repelling in the catalyst layer.

In this regards, Japanese Laid-open Patent Publication No. 2006-286330discloses a method for making an electrode with different hydrophileproperties in a catalyst layer by surface-reforming catalyst particleswith compounds with different hydrophile properties, but this methoddoes not still solve the above problems in an effective way.

DISCLOSURE Technical Problem

Therefore, an object of the present invention is to provide a cathodefor a fuel cell, which may keep moisture in an electrode to a suitablelevel by controlling the degree of water repelling in a catalyst layer.

Technical Solution

In order to accomplish the above object, the present invention providesa cathode for a fuel cell, which includes a gas diffusion layercontacting with a separator having a channel and a catalyst layerinterposed between the gas diffusion layer and an electrolyte membrane,wherein the catalyst layer of the cathode has two portions withdifferent water-repelling properties, and a portion of the catalystlayer that does not face a channel has a higher water-repelling propertythan a portion that faces a channel.

Inventors found that Japanese Laid-open Patent Publication No.2006-286330 discloses that a catalyst layer has regions with differenthydrophile properties, but its effect is insufficient since correlationbetween channel location and moisture control in the catalyst layer isoverlooked. In this reason, as shown in FIG. 5, in the cathode for afuel cell of the present invention, a water-repelling property isenhanced in a portion 22 of a separator 210, which does not faces achannel, namely in a portion of catalyst layers 203, 205 correspondingto a gas diffusion layer 208 pressed by the separator. Thus, moisturemay be more easily discharged to suitably keep moisture in a cell,thereby improving the performance of the cell. In the electrode of thepresent invention, the difference of water-repelling properties betweenthe portion facing a channel and the portion not facing a channel may beconsidered as appropriate when a difference of contact angles to wateris 2° to 20°, but the present invention is not limited thereto.

The portion of the catalyst layer in the cathode of the presentinvention, which faces a channel, may employ a catalyst layer generallyused in the art, and it may include, for example, a metal catalyst or ametal catalyst on a carbon-based support, and a polymer ionomer.

The portion of the catalyst layer in the cathode of the presentinvention, which does not face a channel and has a higherwater-repelling property than the portion facing a channel, may furtherinclude a hydrophobic polymer-based water-repelling enhancer used in theart in addition to a conventional catalyst layer so as to improve thewater-repelling property. For example, this portion may include a metalcatalyst or a metal catalyst on a carbon-based support; a polymerionomer, and a hydrophobic polymer-based water-repelling enhancerselected from the group consisting of polytetrafluoroethylene,polychlorotrifluoroethylene and fluorinated ethylene propylenecopolymer. Here, a weight ratio of the metal catalyst to the hydrophobicpolymer-based water-repelling enhancer may be set to 1:0.02 to 0.40 asnecessary by those having ordinary skill in the art, but the presentinvention is not limited thereto.

In another aspect of the present invention, there is also provided amethod of preparing a cathode for a fuel cell, which includes (S1)preparing a first catalyst layer forming ink and a second catalyst layerforming ink that further contains a hydrophobic polymer-basedwater-repelling enhancer in addition to the first catalyst layer formingink; and (S2) spraying the first and second catalyst layer forming inksto an electrolyte membrane or a gas diffusion layer by means of ink jetspraying such that the first catalyst layer forming ink is sprayed to aportion that faces a channel and the second catalyst layer forming inkis sprayed to a portion that does not face a channel.

In this regards, Japanese Laid-open Patent Publication No. 2006-286330discloses a method for controlling hydrophile properties in a catalystlayer by surface-reforming catalyst particles, but the inventors foundthat this conventional technique needs an additional process forsurface-reforming catalyst particles. However, in the method of thepresent invention, the hydrophobic polymer-based water-repellingenhancer capable of improving water-repelling property of a catalystlayer is added and mixed while the catalyst layer forming ink isprepared, so this method is very simple and does not need any separateprocess.

The first catalyst layer forming ink may adopt any ink generally used inthe art, and for example it may include a metal catalyst or a metalcatalyst on a carbon-based support; a polymer ionomer; and a solvent.The second catalyst layer forming ink may be prepared by further addinga hydrophobic polymer-based water-repelling enhancer used in the art toa conventional catalyst layer forming ink so as to improve thewater-repelling property. For example, the second catalyst layer formingink may include a metal catalyst or a metal catalyst on a carbon-basedsupport; a polymer ionomer; a hydrophobic polymer-based water-repellingenhancer selected from the group consisting of polytetrafluoroethylene,polychlorotrifluoroethylene and fluorinated ethylene propylenecopolymer; and a solvent. Also, a weight ratio of the metal catalyst tothe hydrophobic polymer-based water-repelling enhancer may be set to1:0.02 to 0.40 as necessary by those having ordinary skill in the art,but the present invention is not limited thereto.

In this method, the ink spraying process may be executed in a thermallytreated state if required.

The above cathode may be used for a membrane electrode assembly or afuel cell.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating the principle for generatingelectricity of a fuel cell.

FIG. 2 is a schematic view illustrating a general configuration of amembrane electrode assembly for a fuel cell.

FIG. 3 is a schematic view illustrating that a first catalyst layerforming ink and a second catalyst layer forming ink are respectivelysprayed according to the present invention.

FIG. 4 is a plane view illustrating schematically that a catalyst layeris formed according to the present invention.

FIG. 5 is a cross-sectional view illustrating schematically that acathode according to the present invention is formed.

FIG. 6 is a schematic view illustrating a fuel cell according to anembodiment of the present invention.

MODE FOR INVENTION

Hereinafter, an electrode for a fuel cell of the present invention willbe described in detail according to its preparing method. Prior to thedescription, it should be understood that the terms used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentinvention on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation.

First, a first catalyst layer forming ink and a second catalyst layerforming ink further containing a hydrophobic polymer-basedwater-repelling enhancer in addition to the first catalyst layer formingink are prepared (S1).

The first catalyst layer forming ink according to the present inventionmay be any catalyst layer forming ink used in the art. For example, thecatalyst layer forming ink may include a metal catalyst or a metalcatalyst on a carbon-based support; a polymer ionomer; and a solvent.

The metal catalyst may be representatively any one selected from thegroup consisting of platinum, ruthenium, osmium, platinum-rutheniumalloy, platinum-osmium alloy, platinum-palladium alloy andplatinum-transition metal alloy, or their mixtures, however the presentinvention is not limited thereto.

The carbon-based support may be a carbon-based material, preferably anyone selected from the group consisting of graphite, carbon black,acetylene black, denka black, ketjen black, activated carbon, mesoporouscarbon, carbon nanotube, carbon nano fiber, carbon nano horn, carbonnano ring, carbon nano wire, fullerene (C60) and Super-P, or theirmixtures.

The polymer ionomer may be representatively a nafion ionomer or asulfonated polymer such as sulfonated polytrifluorostyrene.

The solvent may be any one selected from the group consisting of water,butanol, isopropanol, methanol, ethanol, n-propanol, n-butyl acetate andethylene glycol, or their mixtures.

The second catalyst layer forming ink according to the present inventionmay be prepared in the same way as the first catalyst layer forming ink,except that a hydrophobic polymer-based water-repelling enhancer isadded. For example, the second catalyst layer forming ink includes ametal catalyst or a metal catalyst on a carbon-based support; a polymerionomer; a hydrophobic polymer-based water-repelling enhancer selectedfrom the group consisting of polytetrafluoroethylene,polychlorotrifluoroethylene and fluorinated ethylene propylenecopolymer; and a solvent.

The content of the hydrophobic polymer-based water-repelling enhanceradded to the second catalyst layer forming ink may be controlled asnecessary through sufficient experiments. For example, a weight ratio ofthe metal catalyst and the hydrophobic polymer-based water-repellingenhancer may be set to 1:0.02 to 0.40. If the weight ratio ofhydrophobic polymer-based water-repelling enhancer to metal catalyst is0.02 or above, the water-repelling effect is great. However, if theweight ratio exceeds 0.40, an increase rate of water-repelling effect isnot good in comparison to the added amount of hydrophobic polymer-basedwater-repelling enhancer, and mass transfer may be deteriorated. Afterthe ink is prepared as mentioned above, the first and second catalystlayer forming inks are sprayed onto an electrolyte membrane or a gasdiffusion layer by means of ink jet spraying. At this time, the firstcatalyst layer forming ink is sprayed to a portion facing a channel, andthe second catalyst layer forming ink is sprayed to a portion not facingthe channel, thereby forming a catalyst layer (S2).

As shown in FIG. 3, a first catalyst layer forming ink 11 and a secondcatalyst layer forming ink 12 are sprayed onto an electrolyte membrane201 or a gas diffusion layer 208. Ink jet spraying uses the relatedsoftware to adjust a spray location of inkdrops very precisely, and thusindividual inkdrops of the first catalyst layer forming ink 11 and thesecond catalyst layer forming ink 12 may be sprayed onto presetlocations of the electrolyte membrane 201 or the gas diffusion layer208. Thus, as shown in FIG. 4, it is possible to spray the firstcatalyst layer forming ink to a portion 21 facing a channel and to spraythe second catalyst layer forming ink to a portion 22 not facing thechannel.

As shown in FIG. 5, the portion 22 of the catalyst layer, not facing thechannel, receives a pressure by a non-channel portion of a separator 210contacting with a membrane electrode assembly, so pores in the gasdiffusion layer 208 are reduced. If pores in the gas diffusion layer 208are reduced, moisture is not easily discharged, so a moistureconcentration of catalyst layers 203, 205 corresponding to that portionis changed differently from moisture concentrations of other regions.Thus, the catalyst layers 203, 205 of the present invention areconfigured such that the portion 22 not facing the channel has higherwater-repelling property than the portion 21 facing the channel. Thus,the moisture concentration of the portion 22 of the catalyst layers 203,205, not facing the channel, may be kept substantially identically tothe moisture concentration of the portion 21 facing the channel.

The difference of water-repelling properties between the portion facingthe channel and the portion not facing the channel may be measured invarious ways. For example, this difference may be expressed using acontact angle to water. In the present invention, the difference ofwater-repelling properties between the portion facing the channel andthe portion not facing the channel may be considered as appropriate whena difference of contact angles to water is 2° or above, preferably 5° orabove, more preferably 10° or above, but the present invention is notlimited thereto. If the difference of water-repelling properties is toogreat, reactivity in the catalyst layer may be deteriorated. Thus, thedifference of water-repelling properties in the catalyst layer isconsidered as appropriate when the difference of contact angles is 2° to20°.

If the first and second catalyst layer forming inkdrops are sprayed topreset locations on the electrolyte membrane or the gas diffusion layerto be adjacent to each other according to the above method, a catalystlayer is formed. If the first and second catalyst layer forming inkdropsare repeatedly sprayed onto the formed catalyst layer according to theabove method, it is possible to obtain a catalyst layer with a desiredthickness.

After the ink spraying process is completed, a drying process may befurther executed to dry the catalyst layer. In this case, to promote thedrying process of sprayed inkdrops, ink may be sprayed in a thermallytreated state during the ink spraying process.

In the method for making a cathode for a fuel cell according to thepresent invention as described above, fine inkdrops may be sprayed todesired locations by means of the ink jet spraying, so it is possible tocontrol positions of matters included in the ink in the catalyst layer.Thus, though a fuel cell have various channel patterns, the first andsecond catalyst layer forming inks according to the present inventioncan be sprayed to the portion facing a channel or the portion not facinga channel by location adjustment, so it is possible to control regionswith different water-repelling properties in the catalyst layer. Inaddition, it would be easily expected without any experiment by thosehaving ordinary skill in the art that an amount of moisture in anelectrode can be easily controlled by adjusting moisture concentrationof the catalyst layer since the catalyst layer may have differentwater-repelling properties according to the channel location.

The electrode for a fuel cell according to the present invention may beformed on an electrolyte membrane or a gas diffusion layer, so it may beused for manufacturing a membrane electrode assembly for a fuel cellaccording to the present invention.

As shown in FIG. 2, a membrane electrode assembly for a fuel cellaccording to the present invention includes an electrolyte membrane 201;and an anode and a cathode located at opposite sides of the electrolytemembrane 201. The anode and cathode may include a gas diffusion layer208 and catalyst layers 203 and 205, respectively. The gas diffusionlayer 208 for a fuel cell according to the present invention may includesubstrates 209 a and 209 b and microporous layers 207 a and 207 b formedon one side of the substrates 209 a and 209 b, respectively.

The electrolyte membrane employed in the present invention may adopt anyelectrolyte membrane used in the art, for example any one polymerselected from the group consisting of perfluorosulfonic acid polymer,hydrocarbon-based polymer, polyimide, polyvinylidene fluoride,polyethersulfone, polyphenylene sulfide, polyphenylene oxide,polyphosphazine, polyethylene naphthalate, polyester, dopedpolybenzimidazol, polyether ketone, polysulfone, and their acids andbases, but the present invention is not limited thereto.

The gas diffusion layer employed in the present invention may adopt anygas diffusion layer used in the art, and typically may include aconductive substrate made of any one selected from the group consistingof carbon paper, carbon cloth and carbon felt. The gas diffusion layermay further include a microporous layer formed on one side of theconductive substrate, and the microporous layer may be made of acarbon-based material or a fluorine-based resin.

The carbon-based material may be any one selected from the groupconsisting of graphite, carbon black, acetylene black, denka black,ketjen black, activated carbon, mesoporous carbon, carbon nanotube,carbon nano fiber, carbon nano horn, carbon nano ring, carbon nano wire,fullerene (C60) and Super-P, or their mixtures, but the presentinvention is not limited thereto.

The fluorine-based resin may be any one selected from the groupconsisting of polytetrafluoroethylene, polyvinylidene fluoride (PVdF),polyvinyl alcohol, cellulose acetate, polyvinylidenefluoride-hexafluoropropylene copolymer (PVdF-HFP) and styrene-butadienerubber (SBR), or their mixtures, but the present invention is notlimited thereto.

At this time, the catalyst layer is formed on the microporous layer ofthe gas diffusion layer.

The present invention also provides a fuel cell including the membraneelectrode assembly explained above. FIG. 6 is a schematic viewillustrating a fuel cell according to an embodiment of the presentinvention. Referring to FIG. 6, the fuel cell of the present inventionincludes a stack 200, a fuel providing unit 400 and an oxidant providingunit 300.

The stack 200 includes at least one membrane electrode assembly of thepresent invention, and in the case that at least two membrane electrodeassemblies are included, the stack 200 includes a separator interposedbetween the membrane electrode assemblies. The separator prevents themembrane electrode assemblies from being electrically connected to eachother, and transfers a fuel and an oxidant provided from the external tothe membrane electrode assemblies.

The fuel providing unit 400 provides a fuel to the stack 200, and mayinclude a fuel tank 410 for storing a fuel and a pump 420 for providingthe fuel stored in the fuel tank 410 to the stack 200. The fuel may begaseous or liquid hydrogen or hydrocarbon fuel, and the hydrocarbon fuelmay be, for example, methanol, ethanol, propanol, butanol or naturalgas.

The oxidant providing unit 300 provides an oxidant to the stack 200. Theoxidant is typically oxygen, and the oxidant providing unit 300 may be apump for pumping oxygen or air.

INDUSTRIAL APPLICABILITY

The cathode for a fuel cell according to the present invention maycontrol water-repelling property of the catalyst layer differentlyaccording to locations, so it is possible to keep an amount of moisturein an electrode in a suitable way and to restrain generation offlooding, thereby improving the performance of the cell.

1. A cathode for a fuel cell, which includes a gas diffusion layercontacting with a separator having a channel and a catalyst layerinterposed between the gas diffusion layer and an electrolyte membrane,wherein the catalyst layer of the cathode has two portions withdifferent water-repelling properties, and a portion of the catalystlayer that does not face a channel has a higher water-repelling propertythan a portion that faces a channel.
 2. The cathode for a fuel cellaccording to claim 1, wherein the portions with differentwater-repelling properties have contact angles to water, which aredifferent as much as 2° to 20°.
 3. The cathode for a fuel cell accordingto claim 1, wherein the portion of the catalyst layer, which faces achannel, includes a metal catalyst or a metal catalyst on a carbon-basedsupport, and a polymer ionomer.
 4. The cathode for a fuel cell accordingto claim 1, wherein the portion of the catalyst layer, which does notface a channel and has a higher water-repelling property than theportion facing a channel, includes a metal catalyst or a metal catalyston a carbon-based support; a polymer ionomer; and a hydrophobicpolymer-based water-repelling enhancer selected from the groupconsisting of polytetrafluoroethylene, polychlorotrifluoroethylene andfluorinated ethylene propylene copolymer.
 5. The cathode for a fuel cellaccording to claim 4, wherein a weight ratio of the metal catalyst tothe hydrophobic polymer-based water-repelling enhancer is 1:0.02 to0.40.
 6. The cathode for a fuel cell according to claim 3, wherein themetal catalyst is any one selected from the group consisting ofplatinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmiumalloy, platinum-palladium alloy and platinum-transition metal alloy, ortheir mixtures.
 7. The cathode for a fuel cell according to claim 3,wherein the carbon-based support is any one selected from the groupconsisting of graphite, carbon black, acetylene black, denka black,ketjen black, activated carbon, mesoporous carbon, carbon nanotube,carbon nano fiber, carbon nano horn, carbon nano ring, carbon nano wire,fullerene (C60) and Super-P, or their mixtures.
 8. The cathode for afuel cell according to claim 3, wherein the polymer ionomer is nafionionomer or sulfonated polytrifluo-rostyrene.
 9. A method of preparing acathode for a fuel cell, comprising: (S1) preparing a first catalystlayer forming ink and a second catalyst layer forming ink that furthercontains a hydrophobic polymer-based water-repelling enhancer inaddition to the first catalyst layer forming ink; and (S2) spraying thefirst and second catalyst layer forming inks to an electrolyte membraneor a gas diffusion layer by means of ink jet spraying such that thefirst catalyst layer forming ink is sprayed to a portion that faces achannel and the second catalyst layer forming ink is sprayed to aportion that does not face a channel.
 10. The method of preparing acathode for a fuel cell according to claim 9, wherein the first catalystlayer forming ink includes a metal catalyst or a metal catalyst on acarbon-based support; a polymer ionomer; and a solvent.
 11. The methodof preparing a cathode for a fuel cell according to claim 9, wherein thesecond catalyst layer forming ink includes a metal catalyst or a metalcatalyst on a carbon-based support; a polymer ionomer; a hydrophobicpolymer-based water-repelling enhancer selected from the groupconsisting of polyte-trafluoroethylene, polychlorotrifluoroethylene andfluorinated ethylene propylene copolymer; and a solvent.
 12. The methodof preparing a cathode for a fuel cell according to claim 11, wherein aweight ratio of the metal catalyst to the hydrophobic polymer-basedwater-repelling enhancer is 1:0.02 to 0.40.
 13. The method of preparinga cathode for a fuel cell according to claim 10, wherein the metalcatalyst is any one selected from the group consisting of platinum,ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy,platinum-palladium alloy and platinum-transition metal alloy, or theirmixtures.
 14. The method of preparing a cathode for a fuel cellaccording to claim 10, wherein the carbon-based support is any oneselected from the group consisting of graphite, carbon black, acetyleneblack, denka black, ketjen black, activated carbon, mesoporous carbon,carbon nanotube, carbon nano fiber, carbon nano horn, carbon nano ring,carbon nano wire, fullerene (C60) and Super-P, or their mixtures. 15.The method of preparing a cathode for a fuel cell according to claim 10,wherein the polymer ionomer is nafion ionomer or sulfonatedpolytrifluo-rostyrene.
 16. The method of preparing a cathode for a fuelcell according to claim 10, wherein the solvent is any one selected fromthe group consisting of water, butanol, isopropanol, methanol, ethanol,n-propanol, n-butyl acetate and ethylene glycol, or their mixtures. 17.The method of preparing a cathode for a fuel cell according to claim 9,wherein the ink spraying process is executed in a thermally treatedstate.
 18. The method of preparing a cathode for a fuel cell accordingto claim 9, wherein, after the cathode is made, a difference of contactangles to water between the portion facing a channel and the portion notfacing a channel is 2° to 20°.
 19. The method of preparing a cathode fora fuel cell according to claim 9, wherein the electrolyte membrane isany one polymer selected from the group consisting of perfluorosulfonicacid polymer, hydrocarbon-based polymer, polyimide, polyvinylidenefluoride, polyethersulfone, polyphenylene sulfide, polyphenylene oxide,polyphosphazine, polyethylene naphthalate, polyester, dopedpolybenzimidazol, polyether ketone, polysulfone, and their acids andbases.
 20. The method of preparing a cathode for a fuel cell accordingto claim 9, wherein the gas diffusion layer includes a conductivesubstrate selected from the group consisting of carbon paper, carboncloth and carbon felt, a carbon-based material and a fluorine-basedresin.
 21. A membrane electrode assembly for a fuel cell, which includesan electrolyte membrane, and an anode and a cathode respectively formedat opposite sides of the electrolyte membrane, each of the anode and thecathode having a catalyst layer and a gas diffusion layer, wherein thecathode is a cathode for a fuel cell, defined in claim
 1. 22. A fuelcell, comprising: a stack having one or more membrane electrodeassemblies defined in claim 21 and a separator interposed between themembrane electrode assemblies; a fuel providing unit for providing afuel to the stack; and an oxidant providing unit for providing anoxidant to the stack.